Surfactants are widely used in aqueous based personal care, household care and industrial and institutional care formulations as wetting agents, detergents, and emulsifiers. In personal care cleansing products (e.g., shampoos, body washes, facial cleansers, liquid hand soaps, etc.), household care cleaning products (e.g., hard surface cleaners, laundry detergents, dish soaps, automatic dish washer detergents, shower cleansers, bathroom cleansers, car wash detergents, etc.) and industrial and institutional care cleaners (high strength cleaners, detergents, etc.) the surfactant package is one of the most important components in the detersive formulation. These compositions generally comprise a mixture of one or more surfactants as the active detersive ingredient. The surfactant: 1) improves the wettability of the soiled substrate; 2) loosens soil from the substrate; and 3) emulsifies, solubilizes and/or suspends the loosened soil particles in the aqueous wash medium.
Although in principle any surfactant class (e.g., cationic, anionic, nonionic, amphoteric) is suitable in cleansing or cleaning applications, in practice most personal care cleansers and household cleaning products are formulated with anionic surfactants or with a combination of an anionic surfactant as the primary detersive agent with one or more secondary surfactants selected from the other surfactant classes. Anionic surfactants are often used as detersive agents in cleansers and cleaning products because of their excellent cleaning and foaming properties. From the consumer's perspective, the amount and stability of the foam directly relates to the perceived cleaning efficiency of the composition. Generally speaking, the larger the volume of foam produced and the more stable the foam, the more efficient the perceived cleaning action of the composition. Exemplary anionic surfactants traditionally utilized in these formulations include alkyl sulfates and alkyl benzene sulfonates. While the anionic surfactants and in particular the anionic sulfates and sulfonates are efficient detersive agents and have large foam volume and foam stability properties, they are severe ocular irritants and are capable of causing mild to moderate dermal irritation to some sensitized persons. Accordingly, it has become more and more important to consumers that aqueous cleansing compositions are high foaming as well as mild. These combined properties are especially useful if the cleansing compositions are to be topically applied to human skin and hair. Consequently, efforts have been made to provide cleansing products, such as shampoos, bath and shower gels, and facial cleansers that have these properties. The major problem in providing such products resides in the fact that both properties tend to be mutually incompatible. While high foaming detersive surfactants are generally very harsh, mild surfactants tend to provide insufficient foaming properties.
It is known that the irritation caused by anionic sulfates can be reduced by ethoxylation. However, this reduction in irritation is accompanied by a corresponding reduction in foam volume. For example, sodium lauryl sulfate, a high foaming surfactant, causes significant eye irritation. In contrast, sodium laureth-12 sulfate (the corresponding ethoxylate containing 12 ethoxy groups) is almost completely non-irritating, but is a poor foaming agent (see Schoenberg, “Baby Shampoo,” Household & Personal Products Industry 60 (September 1979)). The poor foaming properties of ethoxylated alkyl sulfates are reported in many other publications. For example, U.S. Pat. No. 4,132,678 discloses that the foaming properties of alkyl (C10 to C18) sulfates are drastically reduced if more than 5 ethoxy groups are added to the molecule. Additional attempts to attenuate the adverse irritant effects of anionic surfactants have been made by replacing some of the foam generating anionic surfactant with very mild secondary surfactants. The anionic surfactant is utilized in conjunction with a nonionic and/or an amphoteric surfactant as disclosed in U.S. Pat. No. 4,726,915. However, reducing the amount of anionic surfactant in a cleansing or cleaning composition adversely affects the detersive and foaming properties of the composition.
Another approach for attenuating the adverse irritant effects of anionic detersive surfactants while maintaining high cleansing and foaming properties in personal care cleansing compositions is disclosed in International Patent Application No. WO 2005/023970 to Johnson & Johnson Consumer Companies, Inc. It is disclosed that certain hydrophobically modified materials capable of binding surfactant can be combined with anionic surfactants to produce personal care compositions that exhibit relatively low ocular and/or dermal irritation and maintain relatively high foaming and foam stability properties. Disclosed hydrophobically modified materials include hydrophobically modified crosslinked acrylic copolymers that are synthesized from at least one ethylenically unsaturated carboxylic acid monomer and at least one ethylenically unsaturated hydrophobically modified monomer. The disclosure states that exemplary hydrophobically modified acrylic polymers are set forth in U.S. Pat. No. 6,433,061 to Noveon, Inc. The Johnson & Johnson disclosure additionally exemplifies polymers available under the trade names Carbopol® Aqua SF-1 and Carbopol® ETD 2020 both provided by Noveon Consumer Specialties of Lubrizol Advanced Materials, Inc. as suitable polymers for use as a surfactant binder.
It is to be noted that the polymers disclosed in U.S. Pat. No. 6,433,061 as well as the polymers identified under the Carbopol® Aqua SF-1 and ETD 2020 trade names are crosslinked. At column 5, line 32 of the '061 patent it is disclosed that the “The copolymer of the present invention desirably is crosslinked by a crosslinking monomer.” Moreover, all of the polymers exemplified in the '061 patent disclosure contain a crosslinking monomer. In the trade literature Carbopol® Aqua SF-1 polymer is described in Noveon Consumer Specialties of Lubrizol Advanced Materials, Inc. Technical Data Sheet TDS-294 (July, 2003) as: “ . . . a lightly crosslinked acrylic polymer dispersion designed to impart suspending, stabilizing, and thickening properties to a variety of surfactant-based personal cleansing products;” and Carbopol® ETD 2020 polymer is described in Noveon Consumer Specialties of Lubrizol Advanced Materials, Inc. Technical Data Sheet TDS-187 (January, 2002) as: “ . . . an ‘easy to disperse’ crosslinked polyacrylic acid copolymer processed in a toxicologically-preferred cosolvent system.” The foregoing acrylic based crosslinked polymers are non-linear, branched polymer chains which interconnect to form three dimensional network structures and have long been used in personal care applications for their rheological and structure building properties. Upon neutralization, these water soluble or dispersible polymers possess the unique ability to greatly increase the viscosity of the liquid in which they are dissolved or dispersed, even when present at concentrations considered to be quite low.
As discussed in Johnson & Johnson, supra, the applicants therein disclose a relationship between the critical micelle concentration (CMC) of an anionic surfactant in solution and the tendency of the surfactant to induce irritation. The CMC is illustrated by curve 11 in FIG. 1 of the Johnson & Johnson disclosure. As the surfactant is sequentially dosed into a container (of standardized dimension) of water the surfactant initially occupies the surface (liquid/air interface) of the water/surfactant solution. With each sequential dose of surfactant there is a concomitant reduction in the surface tension of the solution until essentially all of the interfacial surface area is filled. Continued dosing of surfactant results in the formation of micelles within the solution. The surfactant concentration at which the further addition of surfactant does not elicit any appreciable affect in solution surface tension is defined as the CMC (point 12 of curve 11). Additional surfactant added after the CMC point has been found to induce irritation. In contrast, as illustrated in curve 15 of FIG. 1, as anionic surfactant is added to an aqueous solution comprising a hydrophobically modified material, the CMC is shifted to a significantly higher surfactant concentration. Accordingly, the inclusion of hydrophobically modified crosslinked acrylic copolymers allows the use of higher concentrations of anionic surfactant in cleansing and cleaning compositions without the attendant ocular and dermal irritation effects.
Although hydrophobically modified crosslinked acrylic based copolymers have been found to mitigate ocular and dermal irritation of surfactant containing compositions, the use of such polymers can be problematic. As previously discussed, hydrophobically modified crosslinked copolymers are viscosity building agents that increase the viscosity of compositions in which they are dissolved or dispersed. As increasing amounts of viscosity builder are added to a cleansing or cleaning formulation to mitigate the adverse irritation effects of the anionic surfactant there is a corresponding increase in the viscosity of the composition. It is well known in the personal care, household care and industrial and institutional care formulation art that a liquid cleanser or cleaner should have an ideal viscosity. Indeed, viscosity allows for a controlled handling and dispensing of the product during use as compared to a thinner product. In personal care cleansing applications, a thick, rich shampoo or body cleanser is appealing to consumers from a sensory perspective. In household care applications, viscosity permits a better efficacy of the product when applied to non-horizontal surfaces such as toilet bowls, sinks, shower stalls, bath tubs, and the like. In addition, cleansing and cleaning products are expected to be easy to use. In other words, the shear thinning profile of the liquid composition should exhibit high viscosity at low shear conditions and lower viscosity at high shear conditions to aid in the application and removal of the product from the substrate to be cleaned.
However, there are some drawbacks associated with increasing the viscosity of a product beyond its ideal viscosity. Very high viscous products are typically difficult to apply and rinse away, especially if the shear thinning profile of the viscosity building agent is deficient. High viscosities can also adversely affect packaging, dispensing, dissolution, and the foaming and sensory properties of the product. Accordingly, there is a need for an irritation mitigant that does not significantly change the ideal viscosity profile of a surfactant containing composition.